Protective coatings for high strength steels

ABSTRACT

A process for coating a metallic surface of an aircraft. The process includes applying to the metallic surface a composition that polymerizes to form a polyurea having a tensile strength of more than 3500 psi and at least 700% elongation. The polyurea can be formed from an A-side and a B-side, where the weight percents of components for the A-side are: from about 30 to about 65 percent of polyisocyanate; from about 15 to about 70 percent of a polytetramethylene ether glycol; diluent, from 0 to about 20 percent; where the weight percents of components for the B-side are: from 35 to about 40 percent of one or more aromatic diamines; from about 20 to about 70 percent of one or more amine terminated polyether polyols.

This application claims priority to U.S. Provisional Application Ser.No. 60/930,254, filed May 15, 2007, incorporated in its entirety hereinby reference.

Subject to right of the assignee afforded under a Small BusinessInnovation Research (SBIR) program and SBIR Project AF01-131, the U.S.government has a paid-up license in this invention and the right inlimited circumstances to require the patent owner to license others onreasonable terms as provided for by the terms of contract numberFA8650-04-C-5026 which was supported by The United States Air ForceResearch Laboratory at Wright-Patterson Air Force Base.

BACKGROUND OF THE INVENTION

This invention pertains to polyurea coatings for aircraft landing gear.

Landing gear on Air Force C-17 aircraft are subjected to challengingenvironments and abusive situations. Unlike other military transportplanes, the unique design of the C-17 allows it to take off and land onshort, austere runways. This is a tremendous tactical advantage, butunfortunately introduces unforeseen consequences. Unimproved runways areoften laden with foreign object debris that, upon landing or take off,become high-energy projectiles that impact and damage the coating systemon landing gear components. Coating systems for the metallic such aslanding gear that take such punishing abuse are highly desirable.

SUMMARY OF THE INVENTION

The present invention provides a solution to one or more of theproblems, desired concepts, and/or disadvantages discussed above.

In one broad respect, this invention is a steel substrate coated with apolyurea composition that possesses a tensile strength of more than 3500psi and at least 700% elongation. In one embodiment, the polyurea isformed from an A-side and a B-side, where the weight percents ofcomponents for the A-side are: polyisocyanate, from about 25 to about 70percent, preferably from about 30 to about 65 percent; polyether polyol,from about 10 to about 75 percent, preferably from about 15 to about 70percent, more preferably from 30 to 40 percent; diluent, from 0 to about20 percent; for the B-components, from about 0 to about percent of oneor more aromatic diamines, preferably from about 10 to about percent,more preferably with the total amount of such aromatic diamines totalingfrom 35 to about 40 percent; amine terminated polyether polyol, fromabout 20 to about 80 percent, preferably from about 20 to about 70percent, and more preferably 60 percent. If present, a coupling agentcan be used in the B side in an amount from about 0 to 8 percent, moregenerally 0 to 8 percent, and in one embodiment about 2 percent; apigment in an amount of 0 to 10 percent, typically about 2 to 3 percent;and a UV stabilizer in an amount from about 2 to 8 percent, typicallyabout 2 percent.

In one broad respect, this invention is a polyurea coating systemformulated from an A component side that comprises: a diisocyanate, apolyether polyol, and a diluent, and a B component side that comprises:one or more diamines, one or more polyether polyamines, a silanecoupling agent, an optionally pigment, an optional UV stabilizer, and anoptional nanotube component, wherein the polyurea has a tensile strengthof more than 3500 psi and at least 700% elongation.

Advantageously, the coatings of this invention typically possess tensilestrengths of more than 3,500 psi and at least 700% elongation.

In another broad respect, this invention is a process for coating steelcomprising spraying a polyurea coating onto a steel substrate,particularly where the substrate is an aircraft including the landinggear, bay, and belly of the aircraft that are areas that can besubjected to projectile impacts from the ground when the landing gearstrike such projectiles, wherein the polyurea has a tensile strength ofmore than 3500 psi and at least 700% elongation.

The aliphatic polyurea coatings of this invention provide improvedimpact damage and durability for metallic substrates such as aircraftincluding the bay, belly, and/or landing gear. The compositions are freeof VOC and HAP, provide improved sand and rain erosion results at 420mph, possess at least 2500 psi of adhesion to steel, may be colored, aretear resistant, and are superior to existing polyurethane coatings forland gear.

DETAILED DESCRIPTION OF THE INVENTION

Polyurea polymers are polymers which are formed from the reaction of oneor more organic isocyanates with one or more organic polyamines.Polyureas can be formed by bringing the organic isocyanate(s)component(s) in contact with the organic polyamine(s) using for examplestatic mixing equipment, high-pressure impingement mixing equipment,low-pressure mixing equipment, roller with mixing attachments and simplehand mixing techniques, as such techniques are known to those skilled inthe art. Polyurea polymers are useful in caulks, adhesives, sealants,coatings, foams, and many other applications. Specific examples include,but are not limited to, truck-bed liners, concrete coatings, metalcoatings, concrete caulks, roof coatings, decorative coatings, and steelcoatings.

The polyurea elastomer system of the present invention generallyincludes two components, an (A) component and a (B) component. Inparticular, the (A) component includes an aliphatic or aromaticisocyanate. The aliphatic isocyanates employed in component (A) arethose known to one skilled in the art. Thus, for instance, the aliphaticisocyanates are of the type described in U.S. Pat. No. 4,748,192.Accordingly, they are typically aliphatic diisocyanates and, moreparticularly, are the trimerized or the biuretic form of an aliphaticdiisocyanate, such as hexamethylene diisocyanate, or the bifunctionalmonomer of the tetraalkyl xylylene diisocyanate, such as the tetramethylxylylene diisocyanate. Cyclohexane diisocyanate is also considered apreferred aliphatic isocyanate. In addition, the diamines can includecis-1,4-diaminocyclohexane; isophoronediamine; m-xylylenediamine;4,4′-methylenedicyclohexylamine; menthanediamine;1,4-diaminomethylcyclohexane and substituted derivatives thereof.Laromin C-260, available from BASF Corp. is representative of asubstituted 4,4′-methylenedicyclohexylamine derivative. In a mostpreferred embodiment, the cycloaliphatic diaminecis-1,4-diaminocyclohexane, isophoronediamine, and mixtures thereof.Other useful aliphatic polyisocyanates are described in U.S. Pat. No.4,705,814. They include aliphatic diisocyanates, for example, alkylenediisocyanates with 4 to 12 carbon atoms in the alkylene radical, such as1,12-dodecane diisocyanate and 1,4-tetramethylene diisocyanate. Alsodescribed are cycloaliphatic diisocyanates, such as 1,3- and1,4-cyclohexane diisocyanate as well as any desired mixture of theseisomers;1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane(isophoronediisocyanate); 4,4′-, 2,2′- and 2,4′-dicyclohexylmethane diisocyanate aswell as the corresponding isomer mixtures, and the like. Theaforementioned isocyanates can be used alone or in combination.

A wide variety of aromatic polyisocyanates may be used to form theelastomer of the present invention. Typical aromatic polyisocyanatesinclude p-phenylene diisocyanate, polymethylene polyphenylisocyanate,2,6-toluene diisocyanate, dianisidine diisocyanate, bitolylenediisocyanate, naphthalene1,4-diisocyanate,bis(4-isocyanatophenyl)methane,bis(3-methyl-3-iso-cyanatophenyl)methane, bis(3-methyl-4isocyanatophenyl)methane, and 4,4′-diphenylpropane diisocyanate.

Other aromatic polyisocyanates used in the practice of the invention aremethylene-bridged polyphenyl polyisocyanate mixtures which have afunctionality of from about 2 to about 4. These latter isocyanatecompounds are generally produced by the phosgenation of correspondingmethylene bridged polyphenyl polyamines, which are conventionallyproduced by the reaction of formaldehyde and primary aromatic amines,such as aniline, in the presence of hydrochloric acid and/or otheracidic catalysts. Known processes for preparing polyamines andcorresponding methylene-bridged polyphenyl polyisocyanates therefrom aredescribed in the literature and in many patents, for example, U.S. Pat.Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162 and 3,362,979.

Usually methylene-bridged polyphenyl polyisocyanate mixtures containabout 20 to about 100 weight percent methylene diphenyldiisocyanateisomers, with the remainder being polymethylene polyphenyl diisocyanateshaving higher functionalities and higher molecular weights. Typical ofthese are polyphenyl polyisocyanate mixtures containing about 20 toabout 100 weight percent diphenyldiisocyanate isomers, of which about 20to about 95 weight percent thereof is the 4,4′-isomer with the remainderbeing polymethylene polyphenyl polyisocyanates of higher molecularweight and functionality that have an average functionality of fromabout 2.1 to about 3.5. These isocyanate mixtures are known,commercially available materials and can be prepared by the processdescribed in U.S. Pat. No. 3,362,979.

A representative example of a common aromatic isocyanate is methylenebis(4-phenylisocyanate) or MDI. Pure MDI, quasi-prepolymers of MDI,modified pure MDI, etc. are useful. Since pure MDI is a solid and, thus,often inconvenient to use, liquid products based on MDI or methylenebis(4-phenylisocyanate) are used herein. U.S. Pat. No. 3,394,164describes a liquid MDI product. More generally, uretonimine modifiedpure MDI is included also. This product is made by heating puredistilled MDI in the presence of a catalyst.

It is to be understood that the terms “aliphatic isocyanate” and“aromatic isocyanate” also include quasi-prepolymers of aliphaticisocyanates with active hydrogen-containing materials. The activehydrogen-containing materials can include a polyol or a high molecularweight polyoxyalkyleneamine, also described hereinbelow as amineterminated polyethers, or a combination of these materials.

The polyols include polyether polyols, polyester diols, triols, tetrols,etc., having an equivalent weight of at least about 500, and preferablyat least about 1,000 up to about 3,000. Those polyether polyols based ontrihydric initiators of about 4,000 molecular weight and above areespecially preferred. The polyethers may be prepared from ethyleneoxide, propylene oxide, butylene oxide, or mixtures of propylene oxide,butylene oxide and/or ethylene oxide. Other high molecular weightpolyols which may be useful in this invention are polyesters of hydroxylterminated rubbers, e.g., hydroxyl terminated polybutadiene. Hydroxylterminated 3-quasi-prepolymers of polyols and isocyanates are alsouseful in this invention. The polyether polyols that are preferred inthe practice of this invention are the polytetramethylene ether glycols,particularly those having a molecular weight in the range of 500 to5000, preferably about 800 to about 2200, and more preferably from about1000 to about 2000.

Polyamines are used in the practice of this invention to prepare thepolyureas. Such polyamines include aliphatic diamines or aromaticdiamines, and amine terminated polyether polyols (i.e., polyetherpolyamines). The aliphatic and aromatic diamines are sometimes referredto as chain extenders. Combinations of both aliphatic or aromaticdiamines and one or more amine terminated polyether polyols areadvantageously used in the practice of this invention. Especiallypreferred are aromatic diamines and amine terminated polyether polyols.The aromatic diamines useful in this invention include, for example,diethyltoluenediamine (sold commercially as, e.g., UNILINK 4200),1-methyl-3,5-diethyl-2,4-diaminobenzene,1-methyl-3,5-diethyl-2,6-diaminobenzene (both of these materials arealso called diethyltoluene diamine or DETDA and are commerciallyavailable as ETHACURE 100), 1,3,5-triethyl-2,6-diaminobenzene,3,5,3′,5′-tetraethyl-4,4′-diaminodiphenylmethane and the like. Aliphaticdiamines include the chain extenders as described in U.S. Pat. Nos.4,246,363 and 4,269,945. Other diamines include di(methylthio)-toluenediamine or N,N′-bis(t-butyl)ethylenediamine. Cycloaliphatic diaminesthat can be used include cis-1,4-diamino cyclohexane;isophorone-diamine; 4,4′-methylene di-cyclohexylamine; methanediamine;and 1,4-diamino-methyl cyclohexane. Preferably the diamine is anaromatic diamine, more preferably is diethyltoluenediamine,N,N′-dialkylamino-diphenylmethane, or a combination thereof.

Especially preferred are amine terminated polyether polyols, includingprimary and secondary amine terminated polyether polyols of greater than1,500 average molecular weight having from about 2 to about 6functionality, preferably from about 2 to about 3, and an amineequivalent weight of from about 750 to about 4,000. Mixtures of amineterminated polyethers may be used. In a preferred embodiment, the amineterminated polyethers have an average molecular weight of at least about2,500. These materials may be made by various methods known in the art.

The amine terminated polyether resins useful in this invention are, forexample, polyether resins made from an appropriate initiator to whichlower alkylene oxides, such as ethylene oxide, propylene oxide, butyleneoxide, or mixtures thereof, are added with the resulting hydroxylterminated polyol then being aminated. When two or more oxides are used,they may be present as random mixtures or as blocks of one or the otherpolyether. In the amination step, it is highly desirable that theterminal hydroxyl groups in the polyol be essentially all secondaryhydroxyl groups for ease of amination. Normally, the amination step doesnot completely replace all of the hydroxyl groups. However, the majorityof hydroxyl groups are replaced by amine groups. Therefore, in apreferred embodiment, the amine terminated polyether resins useful inthis invention have greater than 50 percent of their active hydrogens inthe form of amine hydrogens. If ethylene oxide is used, it is desirableto cap the hydroxyl terminated polyol with a small amount of higheralkylene oxide to insure that the terminal hydroxyl groups areessentially all secondary hydroxyl groups. The polyols so prepared arethen reductively aminated by known techniques, for example, as describedin U.S. Pat. No. 3,654,370, the content of which is incorporated hereinby reference.

In the practice of this invention, a single high molecular weight amineterminated polyol may be used. Also, mixtures of high molecular weightamine terminated polyols, such as mixtures of di- and trifunctionalmaterials and/or different molecular weight or different chemicalcomposition materials, may be used.

Also, high molecular weight amine terminated polyethers or simplypolyether amines are included within the scope of this invention and maybe used alone or in combination with the aforementioned polyols. Theterm “high molecular weight” is intended to include polyether amineshaving a molecular weight of at least about 2,000. Particularlypreferred are the JEFFAMINE series of polyether amines available fromTexaco Chemical Company; they include JEFFAMINE D-2000, JEFFAMINED-4000, JEFFAMINE T-3000 and JEFFAMINE T-5000. These polyether aminesare described with particularity in Texaco Chemical Company's productbrochure entitled THE JEFFAMINE POLYOXYALKYLENEAMINES.

Other conventional formulation ingredients may be employed in component(A) or (B) as needed, such as, for example, foam stabilizers, also knownas silicone oils or emulsifiers, UV stabilizers, and so on. So-calledmicrotubes can be included. Pigments, for example, titanium dioxide, maybe incorporated in the elastomer system, preferably in the (B)component, to impart color properties to the elastomer, as well as useof matting agents. Reinforcing materials, if desired, useful in thepractice of our invention are known to those skilled in the art. Forexample, chopped or milled glass fibers, chopped or milled carbonfibers, wollostonite, nanotubes, calcium silicate, and/or other mineralfibers are useful.

The composition of this invention can be prepared with a variety ofcomponents in a variety of proportions. In one embodiment, the polyureaof this invention is formed from an A-side and a B-side, where theweight percents of components for the A-side are: polyisocyanate, fromabout 25 to about 70 percent, preferably from about 30 to about 65percent; polyether polyol, from about 10 to about 75 percent, preferablyfrom about 15 to about 70 percent, more preferably from 30 to 40percent; diluent, from 0 to about 20 percent; for the B-components, fromabout 0 to about 65 percent of one or more aromatic diamines, preferablyfrom about 10 to about 50 percent, more preferably with the total amountof such aromatic diamines totaling from 35 to about 40 percent; amineterminated polyether polyol, from about 20 to about 80 percent,preferably from about 20 to about 70 percent, and more preferably 60percent. If present, a coupling agent can be used in the B side in anamount from about 0 to 8 percent, more generally to 8 percent, and inone embodiment about 2 percent; a pigment in an amount of 0 to 10percent, typically about 2 to 3 percent; and a UV stabilizer in anamount from about 2 to 8 percent, typically about 2 percent.

Post curing of the elastomer of the invention is optional. Post curingwill improve some elastomeric properties, such as heat sag. Employmentof post curing depends on the desired properties of the end product.

The (A) component and (B) component of the present polyurea elastomersystem are combined or mixed under high pressure such as using highpressure spray equipment known to on of skill in the art. For example, afirst and second pressurized stream of components (A) and (B),respectively, are delivered from two separate chambers of theproportioner and are impacted or impinged upon each other at highvelocity to effectuate an intimate mixing of the two components and,thus, the formation of the coating system, which is then coated onto thedesired substrate via the spray gun.

The volumetric ratio of the (A) component to the (B) component isgenerally from about 30 to 70 percent to about 70 to 30. Typically,component (A) and component (B) are employed in a 1:1 volumetric ratio.

Advantageously, the (A) and (B) components react to form the presentelastomer system without the aid of a catalyst.

The following examples are exemplary of the invention and not intendedto be limiting as to the scope of the invention and claims hereto.Unless otherwise note, all amounts are by weight.

Example 1 Aliphatic Systems

Coating 1

A small batch of an aliphatic polyurea system was formulated that hadthe following physical characteristics: tack free time 25 seconds, Tg(C)-29.23, tensile (psi) 3348.3, elongation (%) 401.5, tear (Ibf/in) 147(estimated), taber (%) 10A, and gravelometer J400 10A. Coating 1 is alsoprepared with high aspect ratio wollastonite. Another sample of coating1 provided the following physical data: tensile, 3540 psi; elongation726.7%; 0.06 taber %; 125.6 GE impact % elongation; gravelometer (%),0.1; die C tear (lb./in) 512.2. Coating 1 was made using the followingcomponents where the A component and B component were mixed in a 1 to 1ratio by volume.

B Component Weight Percentage Jeffamine 2000 32.87 Jeffamine T-500016.28 IPDA 6.19 Clearlink 1000 41.46 Coupling Agent Z6040 2.1 UVStabilizer 1.11

A Component Weight Percentage IPDI 46.84 Jeffamine 2000 50.38 PropyleneCarbonate 2.79 % NCO 15.59Coating 2

Another aliphatic polyurea system was formulated identical to coating 1except this system was translucent whereas coating 1 was pigmented. Asample of coating 2 provided the following physical data: tensile, 3297psi; elongation 753.1%; 0.11 taber %; 150.2 GE impact % elongation;gravelometer (%), 0; die C tear (lb./in) 490. Coating 2 was made usingthe following components where the A component and B component weremixed in a 1 to 1 ratio by volume.

B Component Weight Percentage Jeffamine 2000 31.29 Jeffamine T-5000 8.86IPDA 6.66 Clearlink 1000 39.88 UV Stabilizer 0.7 Coupling Agent Z60401.54 Pigment Dispersion 11.07

A Component Weight Percentage IPDI 47.43 Jeffamine 2000 52.57 % NCO15.74The components used to make Coatings 1 and 2 were sprayed onto steelsubstrates and subjected to a variety of tests including spraying thecoatings on 1022 mild steel panels, then impacting rocks at 50 miles perhour. This is the Gavelometer J400 rating. Coatings 1 and 2 had a ratingof 10A for this test based on chip and scuff analysis after at least 1pint of gravel was accelerated at the coated metallic substrate. Coating1 shows a weight loss of 0.04% after 200 pints of impacts.

Taber abrasion testing, in general, is conducted by preparing a 4 squareinch coated steel alloy plat having a ½ inch hole in the center toattach the plate to the test instrument. In this procedure, the originalweight of a specimen is measured and recorded, then the specimen isplaced on the abrasion tester. A 500 gram load is placed on top of theabrader wheel and allowed to spin for a set number of revolutions.Different abrading wheels were used: CS17, H-18, and H-22. A hazemeasurement or final weight is taken.

The coatings were subjected to a variety of other tests according tomilitary test specifications, including but not limited to weatherresistance testing such as use of a QUV Accelerated Weathering Testeraccording to Mil-PRF-85285D, fluid resistance tests according toMil-PRF-85284D and possessed excellent tensile retention and elongationretention after extended immersion.

Example 2 Aromatic Polyurea Systems

The following table provides examples of small-scale aromatic polyureaformulations and their properties. In this table, the polyether polyolwas a polytetramethylene ether glycol having a molecular weight of 1000,and the higher molecular weight polyether polyol was apolytetramethylene ether glycol having a molecular weight of 2000.

Weight % ArP-1 ArP-2 ArP-3 ArP-4 ArP-5 ArP-6 A 61 60 56 58 57 57Component Polyether 34 35 34 35 Polyol Polyester 28 35 Polyol Diluent 115.7 8 8 8 8 B Component UNILINK 30 27 27 26 24 33 4200 ETHACURE 7.5 6 85 12 5 100 JEFFAMINE 62.5 67 53.6 69 42.5 62 D2000 JEFFAMINE 11 9 T5000Nanotubes 0.4 0.5 multi-walled (Applied Sciences) Properties Prepolymer410 858 780 870 960 720 Viscosity at 23 C. (cps) Approximate −45 C. to−50 C. to −60 C. to −70 C. to −60 C. to 0 C., −45 C. to Tg Range 10 C.,peak 10 C., peak 10 C., peak 10 C., peak peak at −27 C. 10 C., peak andStorage at −20 C. at −28 C. at −29 C. at −30 C. 490 kPSI at −21 C.Modulus at 390 kPSI 350 kPSI 390 kPSI 575 kPSI 260 kPSI alpha transionYoung's 22,592 20,388 27,845 19668 21,441 26,944 Modulus (psi) Tensile2760 3550 3940 3200 3335 2470 Strength (psi) Elongation 80 240 200 210202 50 (%)

The following table provides examples of aromatic polyurea formulationsthat were scaled up to 5 gallon quantities (5 gallons of each ofComponents A and B) and sprayed. The table includes the properties ofthe sprayed systems. In this table, the polyether polyol was apolytetramethylene ether glycol having a molecular weight of 1000, andthe higher molecular weight polyether polyol was a polytetramethyleneether glycol having a molecular weight of 2000. ETHACURE 100 is amixture of isomers of diethyletoluenediamine. UNILINK 4200 is asecondary aromatic diamine (N,N′-dialkylamino-diphenylmethane) has amolecular weight of about 310. The numbers in parentheses show desirableweight ranges for each of the components in each of the examples ArP-7and ArP-8.

Weight % ArP-7 ArP-8 MDI 56 (40-60) 59 (30-65) Polyether Polyol 32(15-45) Polyether Polyol (higher MW) 36 (20-70) Diluent 8 (0-15) 9(0-20) B Component UNILINK 4200 27 (5-38) 27 (10-48) ETHACURE 100 7(2-18) 7 (0-18) JEFFAMINE D2000 50 (20-70) 49 (20-70) JEFFAMINE T5000 10(0-30) 10 (0-30) Silane Coupling Agent 2 (0-5) 2 (0-8) Pigment 2 (0-10)3 (0-10) UV Stabilizer 2 (2-8) 2 (2-8) Properties Prepolymer Viscosity900 780 at 25 C. (cps) Gel Time 10 seconds 10 seconds Die C TearStrength 660 pli 616 pli Young's Modulus 15,920 psi 12,438 psi TensileStrength 3,866 psi 4,509 psi Elongation (%) 650 707

The materials in the second table were subjected to testing as perExample 1, and possessed excellent impact resistance as well as tensileretention and elongation retention.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as illustrativeembodiments. Equivalent elements or materials may be substituted forthose illustrated and described herein, and certain features of theinvention may be utilized independently of the use of other features,all as would be apparent to one skilled in the art after having thebenefit of this description of the invention.

1. A process for coating a metallic surface of an aircraft, comprising:applying to the metallic surface a composition that polymerizes to forma polyurea wherein the polyurea is formed from an A-side and a B-side,where the weight percents of components for the A-side are: from about30 to about 65 percent of polyisocyanate; from about 15 to about 70percent of a polytetramethylene ether glycol; diluent, from 0 to about20 percent; where the weight percents of components for the B-side are:from 35 to about 40 percent of one or more diamines; from about 20 toabout 70 percent of one or more amine terminated polyether polyols, andwherein the polyurea has a tensile strength of more than 3500 psi and atleast 700% elongation.
 2. The process of claim 1, wherein the metallicsurface is composed of steel.
 3. The process of claim 1, wherein thepolyurea is an aliphatic polyurea.
 4. The process of claim 1, whereinthe polyurea is an aromatic polyurea.
 5. The process of claim 1, whereinthe composition is applied by spraying.
 6. The process of claim 1,wherein the polyisocyanate is hexamethylene diisocyanate, tetramethylxylylene diisocyanate, cyclohexane diisocyanate, 1isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane(isophoronediisocyanate); 4,4′-, 2,2′- and 2,4′-dicyclohexylmethane diisocyanate,or a combination thereof.
 7. The process of claim 1, wherein thepolyisocyanate is formed from methylene bis(4-phenylisocyanate),p-phenylene diisocyanate, polymethylene polyphenylisocyanate,2,6-toluene diisocyanate, dianisidine diisocyanate, bitolylenediisocyanate, naphthalene 1,4-diisocyanate,bis(4-isocyanatophenyl)methane,bis(3-methyl-3-iso-cyanatophenyl)methane, bis(3-methyl-4isocyanatophenyl)methane, 4,4′-diphenylpropane diisocyanate, or acombination thereof.
 8. The process of claim 1, wherein the polyurea isformed using a polytetramethylene ether glycol having a molecular weightof from about 1000 to about 2000 and in an amout from 30 to 40 percent.9. The process of claim 1, wherein the one or more diamines iscis-1,4-diaminocyclohexane; isophoronediamine; m-xylylenediamine;4,4′-methylenedicyclohexylamine; menthanediamine;1,4-diaminomethylcyclohexane; or a combination thereof.